Under ultraviolet radiation, the photocatalytic degradation mechanism of Butyl enzyl phthalate ester (BBP) was studied with TiO2 as the catalyst. As a result, BBP could be degraded efficiently, and the efficiency was 85% after 6h degradation. During the degradation process, the acidity of the degradation system changed obviously. The initial pH of the solution was 6.0, and it increased to 6.23 after 15min degradation, however it decreased to 3.89 after 6h degradation. Some photodegradation intermediates in the different photocatalytic steps were examined and analyzed by means of GC-MS. For the BBP, the photocatalytic degradation firstly reacted on the benzyloxy chain, and new material occured by the recomposition of the educts in the degradation.